The present invention relates to a controllable source of few photons operating at a predetermined wavelength.
A particularly advantageous application of the invention lies in the field of optical telecommunications, in particular high security private telecommunication over short distances.
In the specification below, the term xe2x80x9csource of few photonsxe2x80x9d is used to mean a light source capable of emitting a single photon or a few photons.
In general, conventional optical telecommunications make use of equipment such as laser sources which are designed to enable photons to be used at high concentrations, so as to obtain maximum light power and greatest possible communication distance.
Nevertheless, work is presently under way toward optical communications systems using very low photon fluxes, going down to systems using single photons (see articles by J.P. Goedgebuer, L. Larger, and D. Delorme, Phys. Rev. Lett., 82, 8, 1656, 1999 and by A. Muller, H. Zbinden, and N. Gisin, Europhys. Lett., 33, 335, 1995). Such few-photon devices are particularly desired for studying quantum cryptography, which relies on photon signatures being modified when they are detected, in application of Heisenberg""s uncertainty principle. It is then necessary to be able to manipulate and identify single photons, and consequently to have genuine sources of single photons or to attenuate a flux of photons and work on statistics.
Presently studied sources of few photons are generally based on relatively complicated structures: complex III-V structures (referring to the Periodic Table) with microcavities or very dilute chromophore molecules. However, in any event, such known systems do not give rise to sources of few photons suitable for use in optical telecommunications in order to deploy quantum cryptography.
An object of the present invention is to provide a controllable source of few photons of predetermined wavelength, the source being simple in structure and having a delivery rate that is low and controllable, and capable of operating at ambient temperature at wavelengths that are of interest for optical telecommunications by fiber, in particular in the near infrared around 1.5 micrometers (xcexcm).
This and other objects are attained in accordance with one aspect of the present invention directed to a source that comprises a solid material having a dilute concentration of elements implanted therein that emit light at said predetermined wavelength, an excitation device for exciting said light-emitting elements, and a probe suitable for capturing, by near field coupling, at least one photon emitted by one of the light-emitting elements.
Thus, in the source of the invention, photons are emitted by light-emitting elements implanted in the solid material in a quantity that is known and controlled as a function of the desired concentration. They are then captured by the probe using the physical processes associated with near field optics.
To obtain a source of few photons of the invention, the concentration of light-emitting elements per unit area in the solid material is fewer than 10 elements per square micrometer (xcexcm2). More particularly, the concentration per unit area is fewer than 1 per xcexcm2 for a single-photon source.
The controllable source of few photons of the invention thus makes it possible to implement communications made totally secure by quantum cryptography, whether passing via fiber or via free space without fiber. In the first case, the photon captured by said probe is emitted into free space and then detected by optical transducers. That type of implementation is suitable for short distance communications, of the order of a few tens of meters (m). In the second case, an optical fiber is coupled to said source to convey the captured photon to a detector device. That embodiment can enable communications to be performed between sites that are further apart, up to a maximum radius of 20 kilometers (km), or within business buildings, for example.
Advantageously, the light-emitting element is a rare earth ion selected in particular from the list constituted by erbium, praseodymium, neodymium, and ytterbium. More particularly, the Er3+ ion is selected since its emission wavelength situated at 1.5 xcexcm is in very widespread use in optical fiber telecommunications. The erbium ions are preferably implanted at a dilute concentration in a solid material presenting a very large band gap, in particular electrical insulators, since it is established (Electronics Letters, 25, 11, 718, 1989) that erbium emission at 1.5 xcexcm and at ambient temperature is obtained under excitation greater than 0.8 electron volts (eV), which requires host materials in which the band gap is at least equal to said value.
In practice, said probe is formed by a fine point of size smaller than 1 xcexcm. By way of example, it is constituted by the end of an optical fiber made of glass or of silica.
Finally, the source of few photons of the invention presents the advantage of being suitable for being controlled. To this end, provision is made for it to comprise means for controlling capture by the probe of the emitted photon. These means can be means for controlling the distance between the probe and the light-emitting element.
In conclusion, the controllable source of few photons constituting the subject matter of the invention opens the way to quantum optical communications systems with or without fiber, over short distances and made highly secure by the use of quantum cryptography.